Made DatePart an ExtendedEnum

[DevonFulcher]

Description

Made DatePart an ExtendedEnum so that users don't have to worry about case when entering values into date_part(). Copy-pasted MF comment so that we can remove DatePart from MF.

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] I have signed the CLA
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [x] I have run changie new to create a changelog entry